1. Field of the Invention
This invention relates to an optoelectronic measuring system, particularly to an optoelectronic system for measuring lengths or angles, comprising at least one light source, a scanning unit, which is adjustable relative to a scale member and is connected to a fiber optical system extending from said at least one light source and serving to illuminate a reading unit through the scale on said scale member, and a plurality of optoelectronic receivers spaced from the scale member and connected to the reading unit by another fiber optical system and serving to convert the optical signals received by the reading unit into electric signals.
2. Description of the Prior Art
Such a measuring system has been disclosed in No. EP-A-1 0076 858. That known measuring system serves only to detect angular adjustments and to measure angles. The scale member consists of a slitted plate, which is provided with a coarse incremental scale and is operatively connected to the rotating member to rotate in unison therewith. The slitted plate is illuminated on a light-receiving side at two spaced apart points by two fiber optical lines, which are circular in cross-section and receive light from respective light sources. The illuminating light is transmitted by the slitted plate. On the side which is opposite to the light-receiving side of the plate, a plurality of spaced apart scanning grids are provided and the slitted plate is adapted to emit the light from each of said light sources onto a pair of said scanning grids. The light transmitted by the scanning grids of each pair thereof is transmitted by respective fiber optical lines to an optoelectronic receiver, in which said light is converted to two phase-displaced signals. The receivers and the light sources can be accommodated at a protected location remote from the scanning unit.
No. CH-A-646 784 discloses an incremental length measuring system in which a coarse incremental scale provided on a light-transmitting scale member is illuminated at two spaced apart points with light emitted by respective positive lenses connected to respective fiber optical lines. The light transmitted by the scale member is subsequently transmitted by a scanning grid to light receivers. During the scanning movement, the scanning grid is displaced relative to the scale member. The light receivers collect the received light and transmit it through fiber optical lines to optoelectronic receivers, which generate electric signals which are representative of the illumination and can subsequently be evaluated. In a modified arrangement, the light transmitted by the scale member illuminates a reflecting scanning grid, which is movable relative to the scale member in unison with the adjacent ends of the fiber optical lines, and the fiber optical lines are connected at their other end to light-directing means, which preferably consist of semitransparent mirrors so that the same fiber optical lines can be used to conduct the illuminating light to the scale member and to conduct the reflected light, which has been deflected by the oblique mirrors, to the optoelectronic receivers.
Both known measuring systems comprise nondirective fiber optical lines and the magnitude of the signals generated by the optoelectronic receivers corresponds to the mean illumination of the illuminated area of said optical lines. For this reason the systems can be used only to scan coarse incremental scales. Owing to the use of projecting optical systems, collecting optical systems and scanning grids, all of which must be spaced from the scale member, the scanning unit necessarily has a relatively large overall size and involves a high expenditure. The overall design must exactly be adapted to the measuring system which is employed.
Other scanning units have been used for highly accurate measurements by means of incremental scales having small increments, and for measurements using coding scales, in most cases multi-track absolute scales, which may have an incremental scale track as the scale track having the smallest pitch. In such known scanning units the optoelectronic receivers are mounted close to the scale member on the reading unit which is movable relative to the scale member. Where coding scales are used, coded signals are generated, which correspond to the ratio of the bright and dark areas of the coding tracks in the area being read and said signals are evaluated for the measurement, e.g., for an indication of the measured length or of the measured angle. Incremental scales are used in combination with counting circuits, which count the number of increments traversed from a fixed or selected point of reference on the scale and provide the result of the measurement with a sign that depends on the direction of the scanning movement. In most cases the incremental scale consisting of bright and dark fields is read by means of grids which consist of bright and dark fields and are offset by a non-integral number of grid line spacings. In that case, the scanning movement results in sinusoidal brightness and signal variations and the wavelength of the generated signals equals the scale increment. The zero crossings of said signals can be decoded and counted. Multiplier circuits consisting in most cases of potentiometer circuits may be used for an additional electronic subdivision of the scale increments. Alternatively, the signals generated in response to the scanning movement may be delivered to an interpolating computer, which provides for an even finer subdivision of the scale increments. Suitable evaluating circuits are well known in the art. But such circuits have previously been used or usable only in measuring systems in which the optoelectronic receivers and the associated scanning grids are close to the scale member because otherwise the generated signals cannot be evaluated to give definite results if the incremental scale has very small increments of an order of one-tenth or one-hundredth of a millimeter. The optoelectronic receivers may consist of photodiodes or phototransistors. The scale may be illuminated with incident light, e.g., by means of photodiodes associated with respective scanning grids, or by a common illuminating system, or may be illuminated with transmitted or reflected light. The mode of illumination will depend on the arrangement of the illuminating system relative to the scale and on the optical transmission of the scale member. Other illuminating systems have been proposed in which a common light source delivers light through non-directive glass fiber optical lines to various areas to be illuminated. Such an arrangement provides for a large latitude regarding the mounting of the light source within the scanning unit and even if a relatively high illumination is required will result only in a small temperature rise of the scale member because the heat delivered by the illuminating system can be dissipated.